With trends toward miniaturization of devices, higher performance, and greater circuit density in electrical and electronic parts fields, there is a need for materials with excellent heat resistance, dimensional stability, low moisture absorption, and low dissipation at high frequencies, which is associated with dielectric constant. In particular, as advancements are made in information technology, circuit boards increasingly need to have good performance high frequency.
Circuit boards are commonly made from copper cladding on such reinforcing substrates as follows: glass cloth impregnated with epoxy resin, fluoropolymer film, substrates obtained by impregnation of glass cloth with a liquid in which a polytetrafluoroethylene (PTFE) particles are dispersed as disclosed in Japanese Patent Application Publication Kokai 2001-171038, and laminates obtained by laminating polyphenylene sulfide (PPS) film to a fibrous product mainly comprised of PTFE as disclosed in Japanese Patent No. 3139515.
However, these films and laminates are deficient in the following aspects: Copper-clad laminates obtained by impregnation of glass cloth with an epoxy resin are inferior in high frequency characteristics and moisture absorption characteristics, and they often warp, which is attributable to the differences in the coefficient of thermal expansion of the substrate and the copper foil. Furthermore, they sometimes suffer from an inability to accept plating (copper cladding) when the glass is exposed in the through holes. Through-holes are holes made though the circuit board, the inside of the holes being metal-plated to provide an electrically conductive connection between layers in the board. Copper-clad fluoropolymer laminates also tend to suffer from thermal stress due to the differences in the coefficients of thermal expansion of the copper foil and the fluoropolymer substrate, resulting in problems such as the delamination of the copper foil. Fluorinated film substrates are not easily adhered to: they have difficulty accepting paste and plating during the printing for a circuit formation, lamination of metal foils, or through-hole fabrication. Laminates from PTFE fibrous product and PPS film, while showing low thermal shrinkage, are inferior in high frequency characteristics due to the use of the PPS film, which has a dielectric constant higher than that of the fluoropolymer.
Liquid crystalline polymers (LCP) would be expected to find applications in electronic parts areas because of their high strength, high heat resistance, low coefficient of thermal expansion, and good insulation characteristics. It has been disclosed that blending a melt processible fluoropolymer with an LCP and causing the LCP to be in a fibrous state in the melt processible fluoropolymer matrix can improve the tensile modulus of the melt processible fluoropolymer and its coefficient of linear expansion (EP 1 086 987 A1). It has also been disclosed that introduction of a fluoropolymer having a specific functional group (hereafter called a compatibilizing agent) brings about uniformity in the size of the LCP dispersed phase and dispersion state in the melt mixing stage of the fluoropolymer and the LCP and improves the interfacial adhesion between the fluoropolymer and the fibrous LCP U.S. Patent Application Publication 2001/0006727). However, these fluoropolymer-liquid crystal polymer blends have failed to provide reliable electronic materials and products because during melt extrusion, the LCP molecules extensively orient in the direction of extrusion (machine direction). As a result, the resulting films are highly anisotropic, exhibiting differences in tensile strength and coefficient of linear expansion between the machine direction (MD, the direction in which the LCP fibers are oriented) and the transverse direction (TD, the direction perpendicular to the direction in which the LCP fibers are oriented. In extruded film or sheet, TD is the width of the extrudate).
These shortcomings have prompted a proposal for a process comprising laminating porous fluoropolymer films to both sides of a previously-extruded LCP film, stretching the laminate biaxially under temperature conditions where the porous fluoropolymer is not melted, but where the LCP is melted, thereby reducing or eliminating the anisotropy so as to be able to use the LCP as a circuit substrate material (Kokai H10-34742). This approach is alleged to cause the LCP molecules to be randomly orientated in the plane of the laminate, thereby reducing or eliminating anisotropy in the physical properties. However, unlike conventional thermoplastic polymers, LCPs have rigid molecular chains which tend to slip past one another with essentially no entanglement between individual molecular chains, making it comparatively difficult to stretch them at temperatures below their melting points. At temperatures at or above their melting points, the viscosities of LCPs drop precipitously; and they flow like liquid thereby losing all fibrillar orientation. Therefore, fibrous LCP is very difficult to orient completely randomly, even when the LCP is laminated between porous fluoropolymer films and biaxially stretched.
There is a need for circuit board material that is free from prior art defects.